In general, in order to prepare a hydrocarbon oil from the crude oil, the crude oil is at first subjected to the atmospheric distillation, and then the resulting residual oil is further subjected to the vacuum distillation to be separated into wax-containing hydrocarbon oils with from low to high various viscosities and vacuum distillation residual oils. In addition, the vacuum distillation residual oil is further treated to remove the asphalt component by the solvent deasphalting process so that Brightstock that is wax-containing hydrocarbon oil with the highest degree of viscosity can be produced.
The wax-containing hydrocarbon oils with various viscosities thus obtained become hydrocarbon oils upon a series of treatment stages such as a combination of solvent extraction, hydrogenation refining and dewaxing, or a combination of hydrogenolysis, solvent extraction, hydrogenation refining and dewaxing among other combinations.
The dewaxing process among these production processes described above is the process wherein wax components in a wax-containing hydrocarbon oil are removed and a hydrocarbon oil with low pour point is produced.
Press filtration may be utilized in the process when the dewaxing process is industrially carried out. In this instance, the wax-containing hydrocarbon oil is chilled in the absence of a solvent to separate out wax, and then the wax is subjected to the press filtration. Generally, the dewaxing method with the press filtration process can only treat light type wax-containing hydrocarbon oils because the filtration is limited with viscosity. Thus, a solvent dewaxing method which is capable of treating wax-containing hydrocarbon oils of the light type, the heavy type and the like is universally employed. In the solvent dewaxing method, wax is separated-out and forms a slurry while a wax-containing hydrocarbon oil, a dewaxing solvent and a dewaxing aid are dissolved and chilled. Said slurry is fed to a solid/liquid separator (a filtration apparatus, a centrifugal separator or the like), and a dewaxed oil is obtained by removing the dewaxing solvent upon separation.
Examples of the dewaxing solvent used for the solvent dewaxing method include hydrocarbons (propane, propylene, butane, pentane, and the like), ketones (acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK) and the mixtures thereof, and the like), aromatic hydrocarbons (benzene, toluene, xylene, and the like), and mixtures of ketones and aromatic hydrocarbons (MEK/toluene, acetone/benzene, and the like).
A factor limiting the treatment capacity in the solvent dewaxing process is the filtration rate during the filtration removal of the wax from the slurry, and said rate may be influenced by the crystal structure of the wax separated-out. The crystal structure of the wax separated-out is influenced by the operating conditions in the dewaxing process. Particularly, the condition of the wax separated-out such as the size and the crystal structure of the wax, oil in the crystal, and the like dramatically varies for the same wax-containing hydrocarbon oil depending on the change in the conditions such as the chilling rate, the stirring speed, the chilling temperature, and the like, and thus the filtration rate and the yield of the dewaxed oil are affected. Especially, when the wax-containing hydrocarbon oil is Brightstock, the separation by the filtration has often been running into the problems such as a decrease in the filtration rate, a decrease in the yield of the dewaxed oil, an increase in the pour point of the dewaxed oil due to passing of the fine crystals, clogging of the filter, and the like, because the wax crystals are fine. Various improvements have been achieved in the processes in order to improve the filtration rate and the yield of the dewaxed oil, but the method of adding a dewaxing aid has been carried out as a method for easy control as well as great efficiency. In particular, it has been essential to add a dewaxing aid in a autochilling type dewaxing method such as propane dewaxing.
The prior art technologies such as described below heretofore exist for dewaxing aids. The effect of the mixed use of ethylene vinyl acetate copolymers and polyalkylacrylates or polyalkylmethacrylates is disclosed in JP Patent Publication No. Sho 45-15379, JP Patent Publication No. Sho 49-26922, and JP Patent Laid-open No. Sho 54-11104. The effect of alkylnaphthalene condensation products, or the mixed use thereof with polyalkylmethacrylates is described in JP Patent Laid-open No. Sho 45-15379, JP Patent Publication No. Sho 4946361 and JP Patent Laid-open No. Sho 53-129202. The effect of the use of α-olefin polymers, or copolymers of a-olefin and vinyl acetate is described in JP Patent Laid-open No. Sho 53-121804 and JP Patent Laid-open No. Sho 53-121803. The effect of the use of polyalkylacrylates is described in JP Patent Laid-open No. Sho 404210, JP Patent Laid-open No. Sho 54-123102, JP Patent Laid-open No. Sho 57-30792 and JP Patent Laid-open No. Hei 7-316567. The effect of the use of polyvinylpyrrolidone is described in JP Patent Laid-open No. Sho 55-89392. The effect of the use of copolymers of dialkyl fumarate and vinyl acetate is described in JP Patent Laid-open No. Sho 60-217218 and JP Patent Laid-open No. Sho 61-247793.
Among these prior art technologies, the use as dewaxing aid of a copolymer of a compound having reactive double bonds (a reactive monomer) and vinyl acetate is disclosed in JP Patent Publication No. Sho 49-26922, JP Patent Laid-open No. Sho 54-11104, JP Patent Laid-open No. Sho 53-121804, JP Patent Laid-open No. Sho 53-121803, JP Patent Laid-open No. Sho 60-217218 and JP Patent Laid-open No. Sho 61-247793. Compounds with vinyl acetate groups are decomposed by heat and the like, and acetic acid may be generated. Acetic acid in this case displays corrosiveness against metals such as SUS and the like, to say nothing of iron, and thus their presence is not desirable for the apparatus.
In addition, the use of an alkylnathphthalene condensation product as dewaxing aid is disclosed in JP Patent Laid-open No. Sho 45-15379 and JP Patent Publication No. Sho 49-46361. As such alkylnaphthalene condensation products can be generally obtained through Friedel-Crafts reaction using chlorinated paraffin as the raw material, the complete removal of the chlorine content therein is not easy. Recently, however, there has been a strong demand for chlorine-free products in every field.
Furthermore, the use of polyalkylacrylates as dewaxing aids described in the prior art references shows a good performance under the investigation in the laboratory, but a more effective aid is required since they do not show a good effect, especially, against heavy type wax-containing hydrocarbon oil in the evaluation with the actual apparatus in which the chilling rate during the chilling is 30° C./minute or higher.
The problems to be solved by present invention are that the dewaxing method using the dewaxing aid described in the prior art can not be used for the general purposes depending on the kind of wax-containing hydrocarbon oils used and that the shortcomings (containing of chlorine, corrosion of the apparatus by the product at the time of decomposition, and the like) which can not be avoided due to the structure of these compounds and the production method have to be compensated for. That is to say, in the case of the dewaxing method using a dewaxing aids known heretofore, for instance, a polyalkylmethacrylate, the dewaxing aid alone does not show the effect on both the light type and the heavy type wax-containing hydrocarbon oils, and it is required to further add further compounds, such as alkylnaphthalene condensation products which inevitably contain chlorinated compounds due to their production process and copolymers of reactive monomer/vinyl acetate which may liberate monomeric acids during their decomposition due to their structures, as described above.
The present inventors earnestly investigated in order to solve these problems. As the result, the dewaxing aids whose evaluation under the laboratory scale correlating to the evaluation with the actual apparatus in which the chilling rate during the chilling is 30° C./minute or higher have been found under condition of adding prechilling process (rapid chilling process) just like the actual apparatus to the solvent dewaxing method. They further exhibited the effect against any kinds, from the light type to the heavy type, of the wax-containing hydrocarbon oils, and were found to improve the filtration rate and the yield of the dewaxed oils, as compared with the conventional dewaxing aids.